The present invention relates to a three-tube projection television set in which images from blue, green and red projection cathode-ray tubes are projected and converged by lenses onto a screen to form a color image.
Heretofore, the optical system of the latter type of projection television set has a configuration as shown in FIG. 6.
In the drawing, the reference numeral 10 designates aligned projection cathode-ray tubes. That is to say, a green projection cathode-ray tube 10G is disposed in the center, and a red projection cathode-ray tube 10R and a blue projection cathode-ray tube 10B are disposed to the left and right, respectively, of the green projection cathode-ray tube 10G. These tubes are the outside tubes, as opposed to tube 10G. Three lenses, 20G, 20R and 20B are disposed in front of the green, red and blue projection cathode-ray tubes 10G, 10R and 10B, respectively. Three couplers, 30G, 30R and 30B are provided, each including an image surface correction lens for correcting the curvature of the field caused by the corresponding lens 20. The couplers 30G, 30R and 30B are optically coupled with the front surfaces of the green, red and blue projection cathode-ray tubes 10G, 10R and 10B, respectively.
Images from the three-color projection cathode-ray tubes 10G, 10R and 10B are converged by the lenses 20G, 20R and 20B, respectively, so that a focused color image is projected onto a screen 40.
In the condition described, there is no problem caused by the center lens 20G, because the center lens is disposed with its optical axis 20Gs set at right angles to the screen 40. However, the left and right lenses 20R and 20B are disposed with their optical axes 20Rs and 20Bs inclined relative to the screen 40. Therefore, as shown in FIG. 5, the tube surfaces 10Ra and 10Ba (image surfaces) of the projection cathode-ray tubes 10R and 10B are respectively inclined by angle .beta. relative to the lens surfaces of the lenses 20R and 20B according to a so-called Scheimpflug rule so that an image is focused over the whole surface of the screen 40 in spite of the inclination of their optical axes. The inclination angle .beta. (hereinafter referred to as "shine-proof angle") between the lens surface and the tube surface is represented by the expression ##EQU1## in which m represents the magnification of the lens 20, and .theta.' represents an angle between the lens surface of the lens 20 and the screen 40.
FIG. 4 is a view for explaining a coupler 30 coupled with the whole surface of the projection cathode-ray tube 10. The coupler 30 has a configuration in which a meniscus lens 30b, for correcting the curvature of the field, is mounted through a packing 30c onto the lens side surface of a coupler frame 30a having a cavity in the center thereof, and, on the other hand, a tube surface 10a of the projection cathode-ray tube 10 is closely attached through a packing 30d to the projection cathode-ray tube 10 side surface of the coupler frame 30a. The cavity between the meniscus lens 30b and the tube surface 10a is filled with a liquid solution or the like having the double function of optical coupling and cooling the tube surface 10a. Further, the coupler is disposed so that the optical axis of the meniscus lens 30b coincides with the optical axis of the lens 20.
In this type projection television set, the angle .theta.' between the axis in each of the left and right lenses and the axis of the center lens (the angle .theta.' between the lens surface and the screen) and the magnification m of the lens change corresponding to the variation in projection distance as the screen size (the number of inches) changes. Accordingly, the Scheimpflug angle .beta. is determined in accordance with the screen size.
Conventionally, therefore, various exclusive coupler frames corresponding to various screen sizes have been individually separately produced so that the angle (.theta. in FIG. 4) between a plane to which the meniscus lens is mounted and a plane to which the tube surface is closely attached changes depending on the screen size. In general, the aforementioned coupler frames are, however, produced by aluminum die-cast or the like. Accordingly, various molds must be prepared corresponding to various screen sizes of the projection television sets, so that there arises a problem in cost.